The present invention generally relates to an electrophotographic copying apparatus and more particularly, to an electrophotographic copying apparatus provided with an improved image density control arrangement.
Conventionally, in electrophotographic copying apparatuses, laser beam printers, etc., the following methods have been adopted for controlling density of copied images.
(1) A method in which the revolutions of a developing sleeve or a developing magnetic roll are arranged to be variable.
(2) A method in which the potential of a developing bias, i.e. voltage impressed on a developing electrode is adapted to be variable.
However, in the known method (1) as described above, even when the revolutions of the developing sleeve or developing magnetic roll are arranged to be variable, a sufficient amount of the variation can not be obtained in the copied image density, while there is also an upper limit for the increase of revolutions from the aspect of durability of the developing device also. Moreover, in the case where the revolutions of the developing magnetic roll are adapted to be varied as described above, differences in density tend to take place at the leading edge portion and the trailing edge portion of the copied image (particularly lines and characters having large widths) with respect to the developing direction, i.e. moving direction of the electrophotosensitive member or the photosensitive member (referred to as a photosensitive member hereinbelow) with respect to the developing device.
On the other hand, in the conventional method (2) referred to above, even if the developing bias is arranged to be variable, and the surface potential of the photosensitive member is kept constant, the range of density variation for a line image is small due to the presence of the so-called edge effect, and stable copied images are not available, since the density variation for a line image and that for a solid image are quite different from each other owing to variations of the developing bias.
More specifically, for example, in the case where reversal development is effected as in the laser beam printer, the line image is developed mainly based on the edge effect, and the developing density is in proportion to a value represented by V.sub.0 -Vi, where V.sub.0 is the potential charged on the surface of the photosensitive member and Vi is the surface potential of the image portion. On the other hand, the solid image is developed chiefly based on the electrode effect, and the developing density is in proportion to a value represented by V.sub.B -Vi, where V.sub.B is the developing bias potential. Therefore, the inconveniences as described earlier still take place, even if the potential for the developing bias is arranged to be variable.